Method of operating a cell for the recovery of aluminum by electrolysis of aluminum oxide in a fluoride melt

ABSTRACT

METHOD OF OPERATION O A CELL FOR RECOVERY OF ALUMINUM BY ELECTROLYSIS OF ALUMINUM OXIDE IN A FLUORIDE MELT. IN THE MELT THE VALUES OF INSTANTANEOUS RESISTANCE OF THE CELL ARE CALCUALTED FROM INSTANTANEOUS VALUES OF CELL CURRENT INTENSITY AND CELL VOLTAGE WHICH ARE SAMPLED BY A COMPUTER AT REGULAR INTERVALS OF TIME DURING ELECTROLYSIS, EXCEPT DURING PERIODS ASSOCIATED WITH ANODE EFFECTS AND WITH MANIPULATIONS ON THE CELL, THE ABSOLUTE DIFFERENCE BETWEEN EACH VALUE OF INSTANTANEOUS RESISTANCE AND THE BASE RESISTANCE   OF THE CELL IS ADDED TO A CUMULATIVE TOTAL; A DISTURBANCE IS INDICATED WHEN THE CUMULATIVE TOTAL EXCEEDS A PREDETERMINED VALUE, WHEREUPON THE RESISTANCE OF THE CELL IS RAISED UNTIL THE CAUSE OF THE DISTURBANCE IS ELIMINATED.

I- 1974 K. CHAUDHURI ETAL 3, v

IETHOD 9F OPERATING A CELL FOR THE RBCQVERY OF ALUMINUI BY ELECTRQLYSIS 0F ALUMINUM OXIDE IN A FLUORIDE MELT Filed July 10, 1973 United States Patent 3,829,365 THOD OF OPERATING A CELL FOR THE l QECOVERY OF ALUMINUM BY ELECTROLYSIS OF ALUMINUM OXIDE IN A FLUORIDE MELT Kiranendu Chaudhuri, Gampel, and Peter Bachofner, Liebefeld, Switzerland, assignors to Swiss Alumnuum Ltd., Chippis, Switzerland Filed July 10, 1973, Ser. No. 378,033 Claims priority, application Switzerland, July 18, 1972, 10,751/72 Int. Cl. B01k 3/00; C22d 3/12 US. Cl. 204-67 3 Claims ABSTRACT OF THE DISCLOSURE Method of operation of a cell for recovery of aluminum by electrolysis of aluminum oxide in a fluoride melt. In the melt the values of instantaneous resistance of the cell are calculated from instantaneous values of cell current intensity and cell voltage which are sampled by a computer at regular intervals of time during electrolysis, except during periods associated with anode effects and with manipulations on the cell, the absolute difference between each value of instantaneous resistance and the base resistance of the cell is added to a cumulative total; a disturbance is indicated when the cumulative total exceeds a predetermined value, whereupon the resistance of the cell is raised until the cause of the disturbance is eliminated.

For the recovery of aluminum by electrolysis of aluminum oxide (A1 0 alumina) the latter is dissolved in a fluoride melt, which consists in the greatest part of cryolite Na AlF This melt is contained in a cell having a carbon bottom. The aluminum separated at the cathode collects in liquid state on the carbon bottom of the cell beneath the fluoride melt, and the upper surface of this liquid aluminum in fact constitutes the cathode. Anodes of amorphous carbon dip from above into the melt. Oxygen arises at the anodes by the electrolytic decomposition of the aluminum oxide, and combines with the carbon of the anodes to CO and C0 The electrolysis takes place in a temperature range of about 940 to 975 C.

The principle of an aluminum electrolysis cell with prebaked anodes appears from the single figure of the accompanying drawing. This shows a vertical section in the longitudinal direction through part of a known electrolysis cell.

The steel shell 12, which is lined with a thermal insulation 13 of heat-resisting, heat-insulating material and with carbon 11, contains the fluoride melt (the electrolyte). The aluminum 14 separated at the cathode lies on the carbon bottom 15 of the cell. The surface 16 of the liquid aluminum constitutes the cathode. In the carbon lining 11 there are inserted iron cathode bars 17 transverse to the longitudinal direction of the cell, and these conduct the electrical direct current from the carbon lining 11 of the cell laterally outwards. Anodes 18 of amorphous carbon dip from above into the fluoride melt 10, and supply the direct current to the electrolyte. They are firmly connected via conductor rods 19 and clamps 20 with the anode beam 21. The current flows from the cathode bars 17 of one cell to the anode beam 21 of the following cell through conventional current bus bars, not shown. From the anode beam 21 it flows through the conductor rods 19, the anodes 18, the electrolyte 10, the liquid aluminum 14, and the carbon lining 11 to the cathode bars 17. The electrolyte 10 is covered with a crust 22 of solidified melt and there is a layer of aluminum oxide 23 lying .above the crust. In operation, cavities 25 occur between the electrolyte 10 and the solidified crust 22. Against the side walls of the carbon lining 11 there likewise forms a crust of solid electrolyte, namely a lateral ledge 24. The horizontal extent of the lateral ledge 24 affects the plan area of the bath of liquid aluminum 14 and electrolyte 10.

The distance -d from the lower side 26 of the anode to the surface 16 of the liquid aluminum, also known as the inter-polar distance, can be adjusted by lifting or lowering of the anode beam 21 with the help of the lifting mechanism 27, which is mounted on pillars 28. This alfects all the anodes. An anode can be adjusted individually by releasing the respective clamp 20, shifting the respective conductor rod 19 upwards or downwards relatively to the anode beam 21, and re-tightening the clamp.

Because of attack by the oxygen released during electrolysis, the anodes are consumed continuously on their lower side, by about 1.5 to 2 cms. per day according to the type of cell. At the same time, the height of the liquid aluminum on the bottom of the cell increases continuously by about 1.5 to 2 cms. per day due to the aluminum separated at the cathode.

When an anode has been consumed, then it is exchanged for a fresh anode. In practice, the cell is operated in such a way that, some days after its start of use, the anodes of the cell no longer have the same degree of consumption, and therefore, they must be exchanged separately over a range of several weeks. For this reason, anodes of different starting dates operate together in the same cell, as appears from the drawing.

Because of this complex situation, the interpolar distances d of individual anodes are not exactly equal to each other. It suifices for the purpose of the present invention to consider the average, at any moment in time, of the individual interpolar distances. This average interpolar distance, which itself varies with time, will be termed D.

The principle of an aluminum electrolysis cell with one or more self-baking anodes (Soederberg anodes) is the same as that of an aluminum electrolysis cell with prebaked anodes. Instead of pre-baked anodes, one or more anodes are used which are continually baked from a green electrode paste in a steel jacket during the electrolytic operation by the heat of the cell. The direct current is supplied by lateral steel rods or from above by vertical steel studs. These anodes are renewed as required by pouring green electrode paste into the steel jacket. Adjustments of interpolar distance are made by vertical adjustments of the steel jacket.

By breaking in of the upper electrolyte crust 22 (the crusted bath surface), the aluminum oxide 23 which is above it is brought into the electrolyte 10. This operation is known as servicing of the cell. In the course of the electrolysis. the electrolyte becomes depleted in aluminum oxide. When the concentration of aluminum oxide in the electrolyte falls to somewhere between 1 and 2%, there arises the anode effect, which results in a sudden increase in cell voltage from the normal 4 to 4.5 volts to 30 volts and above. Then at the latest the crust must be broken in, and the A1 0 concentration be raised by addition of new aluminum oxide.

In normal operation the cell is usually serviced periodically, even when no anode eflect occurs. This servicing of the cell will be referred to below as normal servicing of the cell. It occurs for example every two to six hours. In addition upon every anode effect, as explained above, the bath crust must be broken in and the A1 0 concentration raised by addition of fresh A1 0 which corresponds to a servicing of the cell. Thus in operation the anode effect is always associated with a servicing of the cell, which, in contrast to normal servicing of the cell, one can refer to as anode elfect servicing.

The aluminum 14 produced electrolytically, which c01- lects on the carbon bottom 15 of the cell, is generally removed once a day from the cell by conventional tapping devices, for instance sucking devices.

One measurable quantity in the operation of the cell is its base voltage. This depends on the age of the cell, the condition of the carbon lining 1-1, and the composition of the molten electrolyte 10, as well as on the cell current intensity and current density. The base voltage is also affected by the variation of the plan area of the bath in consequence of variation of the horizontal extent of the lateral ledge 24. The base voltage is measured between corresponding points on the anode beams of the cell in question and of the next cell in series. The voltage is the total of the ohmic voltage drops in the parts of the cell through which current flows plus the EMP required for the electrolytic decomposition of the A1 in the electrolyte.

From the base voltage the base resistance of the cell can be calculated according to the following equation:

R is the ohmic base resistance in ohms, U the base voltage in volts, 1.65 the electromotive force in volts and I the cell current intensity in amps.

The correct value of the base voltage corresponds to an optimum average interpolar distance D. In practice the actual interpolar distance is sometimes larger or smaller than it corresponds to the optimum interpolar distance. The departures are substantially produced by increase of the height of the liquid aluminum 14 above the carbon bottom 15, by burning away of the anodes 18 at their lower side 26 and by variation of the dimension of the bath in consequence of variation of the thickness of the lateral ledge 24.

In the ideal case every anode lower side 26 has the same spacing d from the surface 16 of the liquid aluminum 14. However in practice significant differences in the interpolar distance can appear from anode to anode, which can lead to the fact that individual anodes even touch the liquid metal 14 -(local short circuit of the cell). These differences of the interpolar distance of individual anodes of the same cell are caused by defective insertion of new anodes during anode exchange, slipping of the conductor rods 19 as a result of insufficient tightening of the clamps 20, unequal anode quality, bulging of the surface 16 of the liquid aluminum 14 as a consequence of magnetic effects. Large differences of the interpolar distance from anode to anode, especially when it attains local short circuit, lead to heavy disturbances of the cell. =Local overheatings of the electrolyte occur, which have as consequence a lowering of the current efficiency and a rise of the specific electrical energy consumption (kwh./kg. Al). The anodes burn away unequally at their lower side and become powdery at their upper part as a result of burning in air with formation of carbon froth. The powdering of the anodes leads to further raising of the electrolyte temperature and can result in a release of the anodes from their grip on the conductor rods 19.

The problem which has led to the present invention was the recognition in gOOd time of a disturbance of the cell by analysis of the course in time of its base resistance.

According to this invention a cell for the recovery of aluminum by electrolysis of aluminum oxide in a fluoride melt, is operated by a method in which values of instantaneous resistance of the cell are calculated from instantaneous values of cell current intensity and cell voltage which are sampled by a computer at regular intervals of time during electrolysis, except during periods associated with anode effects and with manipulations on the cell, the absolute difference between each value of instantaneous resistance and the base resistance of the cell is added to a cumulative total, and a disturbance indication results from the computer when the cumulative total exceeds a predetermined value which is dependent on the number of intervals per period for a given cell, whereupon the resistance of the cell is raised, by increase of the interpolar distances of the anodes of the cell by a fixed amount, until the cause of the disturbance is eliminated, and finally the original interpolar distances are restored: or if no disturbance indication results within a predetermined period of sampling, then the cumulative total is returned to zero.

Conveniently the predetermined value is expressed in ohms and is in effect the limit of permissible departure from the base resistance of the sum of differences of resistances over each predetermined period.

The time intervals for the sampling of instantaneous values by the computer are chosen at will. Small time intervals of for example 15 to 60 seconds, give statistically more reliable data than large time intervals, of for example 30 minutes and above, for within large time intervals significant alterations in the electrolytic process can usually occur.

The intended alteration of the interpolar distance may be efiectuated either automatically :at the command of the computer, or by hand.

The computer stores the quantities R and R R The special brackets indicate the absolute value of R R independently of the sign of the difference.

The predetermined value of the sum of the absolute differences R- R is an empirical value. With a kA cell, it lies between about 1.5 and 3 microhms for values which are summed up in a period of four hours. It is proportional to the said predetermined period. Advantageously the period for summing up the instantaneous values amounts to at least 1 hour and does not exceed 8 hours.

It has been found for example that the base resistance of a 100 kA cell in undisturbed conditions amounts to 20 to 30 microhms. If the sum of the absolute differences of resistances following one another in time exceeds a limiting value of 2 microhms in a period of four hours, the cell is indicated by the computer as disturbed.

Upon the disturbance indication, the interpolar distance is increased by an amount which in this case produces an increase of the cell voltage of about 0.2 volts. The cell voltage and likewise the interpolar distance are reduced to the original value (before increase of the interpolar distance), when the cause of the disturbance has been eliminated.

For the calculation of the absolute differences of the resistances only those measurements should be taken into account which are not sampled during anode effects or mainpulations on the cell. By manipulations are here to be understood such measures as are necessary from time to time for the correct operation of the cell, in particular the anode exchange, the tapping of aluminum, and the normal servicing of the cell. During anode effects and manipulations of the cell, sampling is not interrupted, but the feeding to the cumulative total is interrupted.

The researches of the inventors have revealed that, for the calculation of the absolute differences of the resistances, preferably those measurements should be excluded which are sampled by the computer up to 15 minutes before and up to 15 minutes after an anode effect, during an anode exchange and up to 5 minutes after the anode exchange, during tapping of aluminum and up to 5 minutes after the tapping of aluminum, as well as during a normal servicing of the cell.

The continuous monitoring of the cell leads to a steady operation, from which results an increase of the current efficiency and a reduction of the specific consumption of energy and anodes.

What is claimed is:

1. A method of operating a cell for the recovery of aluminum by electrolysis of aluminum oxide in a fluoride melt, in which values of instantaneous resistance of the cell are calculated from instantaneous values of cell current intensity and cell voltage which are sampled by a computer at regular intervals of time during electrolysis,

except during periods associated with anode effects and with manipulations on the cell, the absolute difference between each value of instantaneous resistance and the base resistance of the cell is added to a cumulative total, and a disturbance indication results from the computer when the cumulative total exceeds a predetermined value which is dependent on the number of intervals per period for -a given cell, whereupon the resistance of the cell is raised, by increase of the interpolar distances of the anodes of the cell by a fixed amount, until the cause of 1 the disturbance is eliminated, and finally the original interpolar distances are restored; or if no disturbance indication results within a predetermined period of sampling, then the cumulative total is returned to zero.

2. A method according to claim 1, in which those instantaneous values are excluded which are sampled during anode elfects, anode exchange, tapping of aluminum, or normal servicing of the cell.

3. A method according to claim 2, in which instantaneous values are excluded which are sampled up to 15 minutes before and up to 15 minutes after an anode effect, during tapping of aluminum and up to 5 minutes after the tapping of aluminum, as well as during a normal servicing of the cell.

References Cited UNITED STATES PATENTS JOHN H. MACK, Primary :Examiner DONALD R. VALENTINE, Assistant Examiner US. Cl. X.R. 

